The Enhanced Fujita Scale: How Tornadoes are Rated

Apr 10 2015 05:15 AM EDT

weather.com

Practically speaking, it is nearly impossible to measure the actual wind speed inside tornadoes, as they can destroy just about any unprotected weather instruments placed in their path. While ingenious storm chasers and engineers have devised vehicles and instrument packs that can withstand a tornado, the logistics and time involved to deploy such devices before every tornado would be inconceivable.

This is why a measurement system for estimating the intensity of tornadoes after their impact was devised, a system known today as the Enhanced Fujita Tornado Intensity Scale.

The original Fujita scale is named after Dr. Ted Fujita, a University of Chicago severe storms research scientist who came up with the scale in 1971. Dr. Fujita's scale, which ranges from F0 to F5, is based upon the type and severity of damage the tornado produced. At that time, there were very few actual measurements of tornado wind speeds that he could relate to the damage, but he used them -- together with a lot of insight -- to devise approximate wind speed ranges for each damage category.

In subsequent years, structural engineers and meteorologists have examined damage from many tornadoes. They use knowledge of the wind forces needed to damage or destroy various buildings and their components to estimate the resultant wind speeds. The engineers and scientists eventually determined that the original F-Scale wind speeds were too high for categories F3 and higher.

In particular, homes are not built so strongly that 250-300 mph winds are needed to destroy them. In some areas, building codes may only require that roofs withstand 70 mph winds. Once the roof is blown off, walls have less support and begin to fail. In other cases, the house frame may not be properly bolted to the foundation. It then begins to slide off the foundation, perhaps partly lifted by strong upward winds in the tornado. By the early 1990s, even Dr. Fujita recognized these shortcomings and recommended adjusting the assessments.

Guidelines in the original scheme lumped together homes, schools, mobile homes, vehicles, and trees in one short description of damage for each F-Scale category. In the enhanced scheme, detailed descriptions are given for examples of damage to 23 types of buildings and five additional objects like trees, towers, and poles. Wind speed estimates are then provided for each structure and type of damage.

The Enhanced Fujita (EF) Scale was devised by a panel of meteorologists and engineers convened by the Wind Science and Engineering Research Center at Texas Tech University. The Weather Channel's severe weather expert Dr. Greg Forbes was on the team of experts who determined the revised wind speed ranges. Since 2007, the EF Scale has been used to rate tornadoes.

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